Roots: More action down below explains deep soil carbon

Roots are more important than surface biota in creating soil carbon, according to Schmidt et. al's peer-reviewed paper  Persistence of soil organic matter as an ecosystem property (eScholarship, 26-9-2012). "Root-derived carbon is retained in soils much more efficiently than are

above-ground inputs of leaves and needles40–42." Isotopic analyses and

comparisons of root and shoot biomarkers confirm the dominance of

root-derived molecular structures in soil43 and of root-derived carbon

in soil microorganisms44. Preferential retention of root-derived carbon

has been observed in temperate forests45,46, for example, where belowground

inputs, including fungal mycelia, make up a bigger fraction of

new carbon in SOM than do leaf litter inputs44,47. In addition to many

above-ground inputs being mineralized in the litter layer, root and

mycorrhizal inputs have more opportunity for physico-chemical interactions

with soil particles40. At the same time, fresh root inputs may

‘prime’ microbial activity, leading to faster decomposition of older

organic matter48,49 as well as changing community composition50.

Carbon allocation by plants thus plays an important part in soil carbon

dynamics, but it is not known how future changes in plant allocation

will affect soil carbon stocks51."

40. Rasse, D. P., Rumpel, C. & Dignac, M. F. Is soil carbon mostly root carbon?

Mechanisms for a specific stabilisation. Plant Soil 269, 341–356 (2005).

41. Kong, A. Y. Y. & Six, J. Tracing root vs. residue carbon into soils from conventional

and alternative cropping systems. Soil Sci. Soc. Am. J. 74, 1201–1210 (2010).

42. Balesdent, J. & Balabane, M. Major contribution of roots to soil carbon storage

inferred from maize cultivated soils. Soil Biol. Biochem. 28, 1261–1263 (1996).

43. Mendez-Millan, M., Dignac, M. F., Rumpel, C., Rasse, D. P. & Derenne, S. Molecular

dynamics of shoot vs. root biomarkers in an agricultural soil estimated by natural

abundance 13C labelling. Soil Biol. Biochem. 42, 169–177 (2010).

44. Kramer, C. et al. Recent (4 year old) leaf litter is not a major source of microbial

carbon in a temperate forest mineral soil. Soil Biol. Biochem. 42, 1028–1037

(2010).

45. Bird, J. A., Kleber, M.&Torn, M. S. 13Cand15Nstabilizationdynamics in soil organic

matter fractions during needle and fine root decomposition. Org. Geochem. 39,

465–477 (2008).

46. Bird, J. A. & Torn, M. S. Fine roots vs. needles: A comparison of 13C and 15N

dynamics in a ponderosa pine forest soil. Biogeochemistry 79, 361–382 (2006).

47. Godbold, D. L. et al. Mycorrhizal hyphal turnover as a dominant process for carbon

input into soil organic matter. Plant Soil 281, 15–24 (2006).

48. Fontaine, S. et al. Stability of organic carbon in deep soil layers controlled by fresh

carbon supply. Nature 450, 277–280 (2007).

49. Kuzyakov, Y. Priming effects: interactions between living and dead organicmatter.

Soil Biol. Biochem. 42, 1363–1371 (2010).

50. A˚ gren, G. I., Bosatta, E. & Magill, A. H. Combining theory and experiment to

understand effects of inorganic nitrogen on litter decomposition. Oecologia 128,

94–98 (2001).

51. Janssens, I. A. et al. Reduction of forest soil respiration in response to nitrogen

deposition. Nature Geosci. 3, 315–322 (2010).

52. Chabbi, A., Kogel-Knabner, I. & Rumpel, C. Stabilised carbon in subsoil horizons is

located in spatially distinct parts of the soil profile. Soil Biol. Biochem. 41, 256–261

(2009).

53. Jobba´gy, E. G. & Jackson, R. B. The vertical distribution of soil organic carbon and

its relation to climate and vegetation. Ecol. Appl. 10, 423–436 (2000).

54. Trumbore, S. E., Davidson, E. A., de Camargo, P. B., Nepstad, D. C. & Martinelli, L. A.

Belowground cycling of carbon in forests and pastures of EasternAmazonia. Glob.

Biogeochem. Cycles 9, 515–528 (1995).

55. Rumpel, C. & Ko¨gel-Knabner, I. Deep soil organic matter—a key but poorly

understood component of terrestrial C cycle. Plant Soil 338, 143–158 (2011).

A comprehensive overview of key challenges to quantitative understanding of

deep soil carbon.

56. Kalbitz, K., Schwesig, D., Rethemeyer, J. & Matzner, E. Stabilization of dissolved

organic matter by sorption to the mineral soil. Soil Biol. Biochem. 37, 1319–1331

(2005).

57. Torn, M. S. et al. Organic carbon and carbon isotopes in modern and 100-year-old

soil archives of the Russian steppe. Glob. Change Biol. 8, 941–953 (2002).

58. Fierer, N., Allen, A. S., Schimel, J. P. & Holden, P. A. Controls on microbial CO2

production: a comparison of surface and subsurface soil horizons. Glob. Change

Biol. 9, 1322–1332 (2003).

59. Kramer, C. & Gleixner, G. Soil organic matter in soil depth profiles: distinct carbon

preferences of microbial groups during carbon transformation. Soil Biol. Biochem.

40, 425–433 (2008).